Microcavity plasma devices produce a nonequilibrium, low temperature plasma within, and essentially confined to, a cavity having a characteristic dimension d below approximately 500 μm. This new class of plasma devices exhibits several properties that differ substantially from those of conventional, macroscopic plasma sources. Because of their small physical dimensions, microcavity plasmas normally operate at gas (or vapor) pressures considerably higher than those accessible to macroscopic devices. For example, microplasma devices with a cylindrical microcavity having a diameter of 200-300 μm (or less) are capable of operation at rare gas (as well as N2 and other gases tested to date) pressures up to and beyond one atmosphere.
Such high pressure operation is advantageous. An example advantage is that, at these higher pressures, plasma chemistry favors the formation of several families of electronically-excited molecules, including the rare gas dimers (Xe2, Kr2, Ar2, . . . ) and the rare gas-halides (such as XeCl, ArF, and Kr2F) that are known to be efficient emitters of ultraviolet (UV), vacuum ultraviolet (VUV), and visible radiation. This characteristic, in combination with the ability of microplasma devices to operate in a wide range of gases or vapors (and combinations thereof), offers emission wavelengths extending over a broad spectral range. Furthermore, operation of the plasma in the vicinity of atmospheric pressure minimizes the pressure differential across the packaging material when a microplasma device or array is sealed.
Research by the present inventors and colleagues at the University of Illinois has resulted in new microcavity plasma device structures as well as applications. As an example, semiconductor fabrication processes have been adapted to produce large arrays of microplasma devices in silicon wafers with the microcavities having the form of an inverted pyramid. Arrays with 250,000 devices, each device having an emitting aperture of 50×50 μm2, have been demonstrated with a device packing density and array filling factor of 104 cm−2 and 25%, respectively. Other microplasma devices have been fabricated in ceramic multilayer structures, photodefinable glass, and Al/Al2O3 structures.
Microcavity plasma devices developed over the past decade have a wide variety of applications. An exemplary application for a microcavity plasma device array is to a display. Since the diameter of single cylindrical microcavity plasma devices, for example, is typically less than 200-300 μm, devices or groups of devices offer a spatial resolution that is desirable for a pixel in a display. In addition, the efficiency of a microcavity plasma device can exceed that characteristic of conventional plasma display panels, such as those in high definition televisions.
Early microcavity plasma devices exhibited short lifetimes because of exposure of the electrodes to the plasma and the ensuing damage caused by sputtering. Polycrystalline silicon and tungsten electrodes extend lifetime but are more costly materials and difficult to fabricate.
Large-scale manufacturing of microcavity plasma device arrays benefits from structures and fabrication methods that reduce cost and increase reliability. Of particular interest in this regard are the electrical interconnections between devices in a large array. If the interconnect technology is difficult to implement or if the interconnect pattern is not easily reconfigurable, then manufacturing costs are increased and potential commercial applications may be restricted. Such considerations are of increasing importance as the demand rises for displays or light-emitting panels of larger area.
The present inventors have previously developed low cost, large scale arrays and self-patterned formation methods. PCT Publication No. WO 2008/013820, entitled Buried Circumferential Electrode Microcavity Plasma Device Arrays, and Self-Patterned Formation Method, describes microcavity plasma device arrays with circumferential (ring) electrodes that are buried in a thin metal oxide layer and surround the microcavities, while being protected from plasma in the microcavities by a thin layer of metal oxide. The microcavity plasma device arrays can be formed by a self-patterned formation process in which one or more self-patterned metal electrodes are automatically formed and buried in the metal oxide during the anodization process. The electrodes form as a ring around each microcavity, and can be electrically isolated from, or connected to, the ring electrodes associated with adjacent microcavities.
As the area of arrays of microplasma devices and the device packing density (number of devices per unit area) are scaled to larger values, maintaining flatness of the array can become problematic. Stress in such arrays, the result of a mismatch in the coefficients of thermal expansion for the metal and metal oxide, can cause buckling of the entire array structure and distortion in the electrode and microcavity patterns in the arrays. For example, Al and Al2O3 have significantly different coefficients of thermal expansion. Such effects may not present difficulties for array sizes of a few cm2 and device packing densities on the order of 102 cm−2 (or less) but can have a deleterious impact on array performance as the area of the array and the packing density rise.